KR20030014748A - Method and apparatus for echo cancellation in digital communications using an echo cancellation reference signal - Google Patents

Abstract

A new echo cancellation criteria ("ECR") training signal is inserted into the current ATSC 8 VSB data stream to improve echo rejection and maintain compatibility with the ATSC 8 VSB digital ATV standard. The new ECR training signal is also suitable for other multipath or distributed digital communication channels. The pre-equalization subsystem is included in the digital ATV system as the front end of the VSB receiver and is preferably trained with a new ECR training signal. One type of pre-equalization subsystem provides a signal that is fully ATSC compliant at its output, and is therefore particularly useful for standard VSB receivers. Another form of pre-equalization system provides a signal that, in some respects, conforms to the ATSC but retains the ECR signal at its output, and is therefore particularly useful in other applications where signal transfer, analysis purposes, and residual channel information are important.

Description

Method and apparatus for echo cancellation in digital communication using echo cancellation reference signal

Communications engineering must continue to address the problem of restoring signals that have been altered by the communication path over which the signal was transmitted. At least for a parameter that contributes to the signal change, the signal can be restored when the communication path is sufficiently characterized. Thus, the key to the problem of periodically necessary signal recovery is to identify the characteristics of the communication path or channel.

One method for the channel identification problem is to transmit an erase reference signal having a known characteristic on the channel and receive the transmitted signal after the transmitted signal has passed through the channel. The original transmitted signal is compared with the received signal and a model of the channel characteristics is generated based on the comparison. One form of cancellation reference signal useful for correcting echo interference is known as an echo cancellation reference ("ECR") signal. Echo interference, also known as multipath or distributed interference, affects analog and digital communication signals. An example of a system or ECR for improved echo cancellation in an analog television receiver is described in David Koo, US Pat. No. 5,121,211, issued June 9, 1992. Another system and structure for echo cancellation suitable for television is US Patent No. 5,278,872 to Craig B. Greenberg, filed Jan. 11, 1994, and US Patent No. 5,396,299 to the same inventor, filed March 7, 1995. It is described in In addition, the ECR was defined by the United States High Definition Television Systems Committee ("ATSC") and was approved by the following document: August 14, 1992 and revised May 13, 1993, "High Definition Television Systems Committee, Standard A / 49: ghost cancellation reference signal for NTSC ".

Digital high definition television systems ("ATVs") are currently defined in the United States. Features of the digital ATV are documented in several standards of the High Definition Television System Committee (“ATSC”) and are available from NTSC, USA, 29996 Washington DC Suite 1200, K Street N. Double 1750. Basically, the digital television standard describes a system designated for transmitting high-definition video and audio and auxiliary data on a single 6 MHz channel. Such a system can reliably deliver an output of about 19 Mbps on a 6 MHz terrestrial broadcast channel and about 38 Mbps on a 6 MHz cable television channel. Although the RF / transmission subsystem described in the digital television standard is specifically designed for terrestrial and cable applications, the purpose is to allow video, audio, and service multiplex / transport subsystems to be used in other applications. Another general information about the digital television standard is provided in the following publication, October 4, 1995, "High Definition Television Systems Committee, Standard A / 54: A Guide to Using the ATSC Digital Television Standard."

One component of digital ATVs is known as "RF / transmit", which leads to channel coding and modulation. The channel coder adds additional information that can be used by the receiver to take the data bit stream and reconstruct the data from the received signal, which, due to a transmission defect, may not accurately represent the transmitted signal. The modulation (or physical layer) uses a digital data stream to modulate the transmitted signal. One mode of the modulation subsystem is the terrestrial broadcast mode, also known as " 8 VSB ", which uses residual sideband modulation with eight discrete amplitude levels.

As described in more detail in Standard A / 54 above, the VSB signal includes several features that allow the design of a receiver to perform the function of acquiring and fixing the transmitted signal. Signal equalization for channel frequency response and echo uses a training signal and is facilitated by including certain features in Data Field Sync. The use of these features first provides a means for acquiring and synchronizing to the VSB signal, making it more reliable, especially by segment synchronization. The data field sync can then be used to identify itself and further perform equalization of the linear transmission distortion. The VSB signal may be equalized by a database or blind equalization method, which does not use data filter synchronization. The blind equalization method is described in more detail in standard A / 54 described above.

A standard VSB receiver is shown in FIG. 1 and includes a tuner 1, an IF filter 7, an equalizer 9 for equalizing the signal for channel frequency response and echo, a phase tracker 11, a trellis decoder 13. , A data de-interleaver 15, a Reed-Solomon decoder 17, and a data de-randomizer 19. Standard VSB receivers are also available from a variety of manufacturers, including IEEE 1394 / Device Bay ATSC DTV, available from Philips Electronics N.V. A receiver reference design (which is integrated with the Philips type TDA8960 modulator / decoder single-chip device).

Despite the significant advances in current digital communications, echo interference remains one of the most deadly distortions in communications links. Strong echo interference can disrupt all digital communication links. The disruption of the digital communication link is due to an intermediate distortion scenario called intersymbol interference ("ISI"), which results from the interaction between the echo state and the transmitted data itself.

Several proposals and methods are known or have been put to practical use in order to reduce the impact of echo interference using equalizers based primarily on statistical data information obtained from adaptation and derived from multipath or distributed communication channels. Some of these proposals and methods are described in the following paper, that is, in June 1995, in the Journal of the Institute of Motion Picture and Television Engineers, David Koo, "ITU System-C Standard Ghost Canceling Reference Signal." Nevertheless, current ATSC 8 VSB digital ATV systems are strongly affected by echo interference, although not limited to noise. Therefore, it is desirable to improve the performance of ATSC 8 VSB digital ATV systems as well as other similar communication systems when echo is present while maintaining its advantages in white noise environments.

The present invention relates to a method and apparatus for echo cancellation in digital communication using an echo cancellation reference signal.

1 is a block diagram of a conventional VSB receiver.

2 is a block diagram of a digital ATV system integrating with a pre-equalization subsystem that removes an ECR signal after equalization.

3 is a schematic diagram of an ATSC data system for the digital ATV system of FIG. 2 including echo cancellation reference signals at various locations.

4 is a block diagram of a digital ATV system integrating with a pre-equalization subsystem that does not remove the ECR signal after equalization.

5 is a schematic diagram of an ATSC data stream for the digital ATV system of FIG. 4 including echo cancellation reference signals at various locations.

6 is a block diagram of an equalizer suitable for the digital ATV system of FIGS. 2 and 4.

FIG. 7 illustrates an exemplary waveform of the signal function versus time from the class of echo cancellation reference signal suitable for the ATSC data streams of FIGS. 3 and 5.

8 shows an exemplary waveform of a flat spectral plot from a class of echo cancellation reference signals suitable for the ATSC data streams of FIGS. 3 and 5.

An embodiment of the present invention includes an extraction circuit for extracting a copy of an echo cancellation reference signal from a digital data stream transmitted on a transmission path; A microprocessor coupled to an extraction circuit for calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And a filter receiving the filter coefficients and receiving the digital data stream at the input and coupled to the microprocessor to substantially cancel echo interference from the digital data stream using the filter coefficients. The echo cancellation reference signal is non-cyclic, has a substantially flat frequency response within the bandwidth of the transmission path, has very many amplitude peaks in time intervals, and shorter tails proportional to very many amplitude peaks. Has

Another embodiment of the present invention provides digital data comprising a plurality of field syncs occurring on a transmission path, a segment of a plurality of symbol spaces between successive field syncs, and a plurality of echo cancellation reference signals between field syncs. A device for substantially canceling echoes occurring during transmission of a stream. The apparatus comprises means for receiving a digital data stream after transmission; Means for extracting a copy of an echo cancellation reference signal from the received digital data stream; Means for calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And means for substantially canceling echo interference from the received digital data stream using the filter coefficients. The echo cancellation reference signal is non-cyclic, has a substantially flat frequency response within the bandwidth of the transmission path, has a large number of amplitude peaks in a time interval, and a shorter tail in proportion to the number of amplitude peaks. Has

Yet another embodiment of the present invention is an apparatus for modulating an ATSC VSB data stream prior to transmitting to a receiver to support enhanced echo cancellation at the receiver, wherein the ATSC VSB data stream includes multiple field synchronizations and continuous field synchronizations. A segment of a plurality of symbol spaces in between, said apparatus comprising means for inserting a plurality of echo cancellation reference signals between said field syncs, said echo cancellation reference signal being generally a signal defined by the following equation: Is the class of,

Where A, b and Ω are real.

Yet another embodiment of the present invention provides digital data comprising a plurality of field syncs on a transmission path, a plurality of symbol space segments lying between successive field syncs, and a plurality of echo cancellation reference signals between the field syncs. A method for substantially eliminating echoes occurring while transmitting a stream. The method includes receiving a digital data stream after transmission; Extracting a copy of the echo cancellation reference signal from the received digital data stream; Calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And substantially canceling echo interference from the received digital data stream using the coefficients. The echo cancellation reference signal is non-cyclic, has a substantially flat frequency response within the bandwidth of the transmission path, has a large number of amplitude peaks in time intervals, and has a shorter tail in proportion to the large number of amplitude peaks. Have

Yet another embodiment of the present invention is a method for modulating an ATSC VSB data stream before transmitting to the receiver to support an enhanced echo cancellation function of one receiver, wherein the ATSC VSB data stream is continuous with multiple field synchronizations. And a plurality of symbol space segments between the field syncs, the method comprising inserting a plurality of echo cancellation reference signals between field syncs, the echo cancellation reference signal being generally defined by the following equation: Is a class of,

Where A, b and Ω are real.

New Echo Canceling Criteria ('ECR') training signals are inserted into current ATSC 8 VSB data streams to improve echo rejection and maintain compatibility with the ATSC 8 VSB digital ATV standard. One type of pre-equalization subsystem provides a signal that is fully ATSC compliant at its output and is therefore particularly useful for standard VSB receivers. At some point in its output it provides a signal that conforms to the ATSC but includes an ECR signal, and is therefore particularly useful for signal transmission, analysis, and other applications where the remaining channel information is important. It improves the performance of ATV systems while maintaining its benefits in white noise environments. The system is expected to be able to process an echo in the range of ± 80 μs and -10 dBc.

An example of a digital ATV system 20 that is integrated with a pre-equalization subsystem that generates a signal that is fully ATSC compliant at its output is shown in FIG. 2. The improved performance of the digital ATV system 20 is performed at the transmission bit rate of 10.76 Msamples / s to 10.98 Msamples / s. Digital ATV system 20 includes a standard VSB receiver 40, such as the VSB receiver shown in FIG. Moreover, the leading equalizer 32 in the pre-equalization subsystem 30 is disposed in front of the VSB receiver 40. The ATSC data stream containing the appropriate ECR signal is input to the leading equalizer 32 instead of the VSB receiver 40. The tip equalizer 32 preferably has a relatively long time length to completely cover the entire echo range. Several adaptation / initialization schemes are usefully used to train the equalizer 32, including known schemes such as, for example, correction and precision channel model measurements.

The leading equalizer 32 is preferably trained on the new ECR signal so that the output of the leading equalizer 32 is in the vicinity of the clear signal where echo is substantially eliminated. The added ECR signal deviates from the data stream in the temporary memory following the leading equalizer 32 using known techniques so that the final signal is a signal that is ATSC compliant. Although many other types of memory may be suitable, FIG. 2 shows FIFO register 34 for this purpose. This final signal is fed to the output of the FIFO register 34 and passed to the standard VSB receiver 40. It can be seen that the leading equalizer 32 and FIFO register 34 can be implemented in discrete integrated circuits or as an integrated solution on a single chip.

Modifying the ATSC data stream for the digital ATV system 20 is shown in data stream 50 of FIG. 3. In the same baseband data symbol stream of the current ATSC 8 VSB standard, a combination of blank symbol spaces and periods of the new ECR signal is inserted between field syncs, eg field sync 51, 54. The combination of the periods of the blank symbol spaces and the ECR signal is for convenience called a macro assembly ("MEA"). Blank symbol spaces are used in the MEA to prevent contamination of the ECR due to post and pre echo. In the representation of the ATSC data stream shown in FIG. 3, MEA signals, eg, MEA signals 52, 55 are illustratively inserted into places immediately after field syncs. Several places have to be tried in practice, since special places can be advantageous under certain conditions, especially in the case of higher transient speed coverage to handle faster and dynamically changing echoes. Advantageously, the insertion of the MEA signal in the current ATSC VSB data stream does not reduce the 312 segment used between field syncs.

The ECR in the MEA is a high performance echo cancellation reference signal, which is a mathematically formulated high energy and high performance signal with a white spectrum. The ECR must also be specifically tailored to the expected digital data transmission conditions and to receive the least undesirable interference in the channel feature process compared to known signals applied as current echo reduction criteria. Examples of known signals applied as current echo reduction criteria include pseudo-random columns, tuner columns, and windows sin (x) / (x). Examples of undesirable interference include contamination and correction from noise (not limited to additive white Gaussian noise ("AWGN") only), magnetic echo-induced data symbols, co-channel interference and its beating results, adjacent Channel signal and its beating results, and its own in-channel intermodulation results. Generally speaking, the proper MEA adjustment to the system before is to support, improve and guarantee the required performance of the ECR described above.

MEA overhead is estimated to be about 2% of the total system's resources. The MEA overhead thus assessed is based on the echo delay and its transient behavior under current terrestrial data communications and broadcast conditions, based on the currently available echo field inspection statistics, as obtained from the National Association of Broadcasters' echo inspection reports. It is expected to fit the range. The measured MEA overhead changes as new echo field check progress is achieved.

The echo cancellation / reduction range is measured to be about −80 μs to +82 μs, which is based on currently available echo field inspection statistics. These echo field test statistics are from +51 μs to +62 μs echo with about 30% voltage intensity in Sydney, Australia; In our cities, finding relatively long echoes is very easy, such as +61 μs to +62 μs echoes with 30% voltage intensity in New York City, USA. Therefore, echoes from +70 μs to +80 μs with large amplitudes are believed to exist in New York City, and similar results are believed to be found in other cities. Also, if long racking echoes, such as +60 μs to +80 μs, are present with large amplitudes, leading echoes with the same time width can be generated because the receiver antennas are not very carefully placed by the average consumer. I think. This means that leading echoes are very likely to exist in large cities from -61 μs to a much longer reading position in time.

Based on the inspection and analysis described above, a good terrestrial digital communication / broadcasting standard and its hardware receiver system have a reading of -80 μs to racking +80 μs, to provide reliable reception in "every place" in poor echo environments and conditions And you should be able to reject echoes that are a bit longer. The digital ATV system 20, which is the system of FIG. 2, is expected to perform echo rejection in the range of -80 μs to +82 μs with large amplitude, with MEA overhead of about 2% of the total system stream of the current ATSC standard. Limited to Advantageously, the current ATSC standard does not change with respect to its original payload data rate and / or current data arrangement. This keeps the decoding chip compatible and avoids the need to retest the performance of new arrays of all symbols, bits, interleaving, error correction code, payloads, etc., resulting in relatively faster hardware realization and relatively simpler and faster system checks. Make it possible.

In order to implement the digital AV system 20 of FIG. 2 without changing the data payload and other bit configurations, a viable transmission solution is to shorten the symbol time duration by about 2%. As shown in FIG. 3, the 832 symbols of each segment of the data stream 50 are shortened to about 76 μs. As a result, the baseband bandwidth is correspondingly increased by about 2% from the original bandwidth of 5.38 MHz to the modulated bandwidth of 5.49 MHz. However, the bandwidth increase in such a scheme is so small that this new bandwidth is supported by both the RF, baseband and SAW filters at the current receiver front end. For example, the new bandwidth is supported by the tuner 1, IF filter and sync detector 3, and NTSC reject filter 7 of the conventional VSB receiver of FIG. Preferably, the original Square Root Raised Cosine (SQRC) filter standard (with 11.5% roll-off) can only be changed by a very small amount. As long as SQRC mathematically retains the characteristics of SQRC, data recovery performance is not changed.

The MEA signals 52, 55 are removed from the substantial echo free data stream of the output of the leading equalizer 32 by the FIFO 34 before the data stream is provided to the VSB receiver 40. This is shown in FIG. 3 where the substantial echo free ATSC flexible data stream 57 includes field synch 51, 54 as well as 312 segments in regions 53, 56, but includes MEA signals 52, 55. I never do that. After re-clocking to store 832 symbols in each segment at 77 μs, the substantial echo free NTSC data stream 57 without ECR or MEA and fully adapted to the current ATSC 8 VSB standard is pre-equalized sub It can be used at the output of system 39 and is adapted by decoding with existing ATSC 8 VSB chips and systems.

At about 2% MEA overhead, the ECR insertion rate is 41 ECRs per second, which interferes with the field sync (FS) of the current ATSC 8 VSB. Based on this ECR insertion rate, according to Nyquist sampling theory, the speed of the fastest dynamic echo that can be canceled or rejected is about 20 Hz. This 20 Hz dynamic echo rejection rate is thought to cover many and possibly most reception conditions due to dynamic echo pattern changes. However, statistically, a double speed ECR and MEA can be inserted when a higher dynamic echo rejection rate is clearly needed. For example, two MEA signals (not shown) may be inserted about 156 segments away from each other and between two field syncs. This brings a fast dynamic echo rejection rate to 41 Hz despite the disadvantage of raising the MEA overhead to about 4%. While these results are progressively contrasted with one of the major benefits of using the current ATSC 8 VSB system, the so-called high data payload with very little overhead (high efficiency) compared to many other systems, It can be used when dynamic echo rejection is required.

The main advantage of digital ATV systems integrating with pre-equality subsystems is the compatibility of existing systems and chips and subsystems. In other words, many existing ATSC 8 VSB systems from many manufacturers can be reused. One possible implementation for supporting chip reuse is independent such that the pre-equalization subsystem 30 (FIG. 2) is placed directly in front of the current receiver chip with many pre-equalization subsystems 30 (FIG. 2). It can be designed as a self-contained chip or module. This is possible because, assuming under normal to severe echoing conditions with reasonable incidence SNR of about 27 to 28 dB, the leading equalizer 32 is measured to provide more than 95% of the echo in the incident data stream-perhaps 98 This is because% or more is removed. After the echo cancellation process, the ATSC data stream is sufficiently echo canceled and easy to process and decode.

An example of a digital ATV system 60 that is integrated with a pre-equalization subsystem that does not remove ECR and MEA is shown in FIG. 4. Performance improvement of the digital ATV system 60 is achieved with a slight reduction in the data payload. Digital ATV system 60 includes a modulated VSB receiver 80. Pre-equalization subsystem 70 with leading equalizer 72 is disposed in front of VSB receiver 80. The ATSC data stream is input to the leading equalizer 72 instead of the VSB receiver 80. The leading equalizer 72 is generally similar to the leading equalizer 32 (FIG. 2) and is preferably trained in the same way in the new type of ECR signal so that the output of the leading equalizer 72 is nearly clear with substantially no echo. It is a signal. Tip equalizer 72 may be implemented on discrete integrated circuit chips or may be an integrated solution within a single chip.

Unlike the pre-equalization subsystem 30 shown in FIG. 2, the pre-equalization subsystem 70 of FIG. 4 does not include a FIFO register. The entire stream of signals and data can be processed without removing ECR and MEA from the data stream so that the data can be decoded directly. This is possible because in some applications the residual channel information is still performed by the embedded ECR and is very useful information for signal connection, analysis purposes, etc.

Modulation of the ATSC data stream for the digital ATV system 60 is shown in the data stream 90 of FIG. 5. The combination of the blank symbol space and the period of the new ECR signal-MEA- is inserted somewhere in the data symbol stream, as in the data stream 50 shown in FIG. However, the MEA signal is inserted into place in each of the 312 segment regions. For example, FIG. 5 shows two 312 segment regions 93 and 96, which include respective MEA signals 92 and 95 at positions immediately after each field sync 91 and 94.

The MEA overhead is estimated to be about 2% of the resources of the overall system, as in the data stream 50 shown in FIG. The echo cancellation / reduction region is evaluated to be at least -80 μs to +82 μs, as in the digital ATV system 20 shown in FIG. 2. However, unlike the modulation for the ATSC data stream 50 shown in FIG. 3, the modulation for the ATSC data stream shown in FIG. 5 does not include changing one of the current data framing structures of the ATSC data stream. . In particular, the symbol duration does not change. This means that although the data payload is reduced by 2%, the overall bandwidth of the data signal is maintained. Instead of the original 19.28 Mbit / s of the current ATSC 8 SVB standard, the data payload is reduced to 18.90 Mbit / s. This slightly lower data payload can be easily absorbed by proper program management. Advantageously, all filters in the receiver 70, from transmit bandwidth and RF and IF filters to baseband filters, are unaffected at all and have problems derived from VSB receivers or even slightly in the bandwidth arrangement of the system. It will not be modulated.

However, the VSB receiver 70 includes some basic variations on the standard VSB receiver as follows. First, VSB receiver 70 may identify and skip the MEA signal in data stream 90. Secondly, the VSB receiver can handle any breakup in conventional code so that the twelve sequential switching sites work properly. Third, VSB receiver 70 may pick up the data of the data stream and link it to the previous state. Due to the replacement of the data with the MEA signal, it is likely that any image data will be lost and therefore the data and convolutional code will be discontinuous. The transmitter and receiver must continue the convolutional code from the old data to the new data stream. Fourth, VSB receiver 70 has a modified data interleaver that can maintain order when convolutional codes become discontinuous.

At about 2% MEA overhead, the ECR insertion rate is 41 ECR per second, as in the data stream 50 shown in FIG. In statistical fact, when higher dynamic echo rejection rates are clearly needed, the doubled speeds ECR and MEA can be inserted, as in the data stream 50 shown in FIG.

Tip equalizers 32 and 72 are suitably designed, many suitable designs being known in the art. An example of an equalizer 100 suitable for use with the tip equalizers 32, 72 is shown in FIG. The received digital ATSC data stream containing the MEA signal is supplied to the FIR filter 102 and the IIR filter 104. The ECR signal in the MEA signal from the received digital ATSC data stream is placed into buffer 108 and fed to a microprocessor or other suitable logic 106. The microprocessor 106 comprising a memory (not shown) containing a version of the appropriate ECR transmitted compares the contents of the buffer 108 with the appropriate version of the transmitted ECR, models the impulse response of the channel, and reverse channel. Calculate the column of coefficients for filters 102 and 104 that perform the characteristic. These filter coefficients are fed to filters 102 and 104 to compress the existing echoes, which results in an ATSC data stream at the output of filter 104 where the echo component is substantially reduced.

A suitable class of ECR signal for performing the task of probing multipath and distributed digital communication channels is defined by the following equation. In other words,

(One)

Here, A, b and Ω are all parameters that are changed or adjusted to modify or change the characteristics of the signal in its entire family of functions. In digital communication, the Ω parameter is determined to be equal to the Nyquist frequency of the sampling rate when the function's waveform is discontinuously sampled when the function is used as an echo cancellation reference signal in the digital communication. Parameters A and b have a high energy and are selected to set the time signal to be compressed in time. An exemplary set of values for the parameters of equation (1) is as follows. That is, A = 0.707 x 8.6 x 10 ^-6 volts; b = 0.9 x 10 ^-12 sec ² / radian; Ω = 5.38 x 2πx 10 ⁶ radian / sec.

In general, ECR signals of this signal class have the following characteristics. That is, (1) high signal energy level; (2) absolute white spectrum; (3) non-cyclic features; (4) the shortest time duration at a given amplitude and a given required signal energy level; (5) a longer time domain signal patching length than the signal itself that does not affect its spectral whiteness characteristics; (6) an auto-correction function, as a single δ function, especially for echo rejection in digital communications; (7) successive group delay functions within the band of interest; And (8) based on the features " 2 " and " 6 " The signal defined in equation (1) satisfies all the specified features "1" to "8".

An illustrated waveform of this time signal function is shown in FIG. 7. The characteristic of this class of signal as a function of time is non-cyclic, which is generally a flat response, a longer signal with much more peaks than the conventional signal and proportionally shorter tails.

8 shows a flat spectral plot of the signal defined by equation (1), which implies the features 1, 2, 3 and 5 described above directly and indirectly.

The scope of the invention is set forth in the following claims. The description of the various embodiments described herein is illustrated with respect to the invention and is not intended to limit the scope thereof. Many modifications and variations of the embodiments described herein will be apparent to those skilled in the art upon reference to this document and modifications and equivalents of the various elements of these embodiments will be apparent to those skilled in the art. It is to be understood that such and other variations and modifications disclosed herein are possible without departing from the scope and spirit of the invention as described in the following claims.

Claims (20)

In the equalizer, Extraction circuitry 108 for extracting a copy of the echo cancellation reference signal from the digital data stream transmitted on the transmission path; A microprocessor (106) coupled to an extraction circuit for calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And A filter (102, 104) coupled to a microprocessor that receives the filter coefficients and receives the digital data stream at an input to substantially cancel echo interference from the digital data stream using the filter coefficients; The echo cancellation reference signal is non-cyclic, has a substantially flat frequency response within the bandwidth of the transmission path, has very many amplitude peaks in time intervals, and has a shorter tail in proportion to very many amplitude peaks. Equalizer. The method of claim 1, The echo cancellation reference signal is a class of signal defined by Where A, b and Ω are real equalizers. The method of claim 2, A = 0.707 x 8.6 x 10 ^-6 volts; b = 0.9 x 10 ^-12 sec ² / radian; And equalizer with Ω = 5.38 x 2πx 10 ⁶ radian / sec. Echoes occurring during transmission of a digital data stream comprising a number of field syncs occurring on the transmission path, segments in a plurality of symbol spaces between successive field syncs, and a number of echo cancellation reference signals between field syncs An apparatus for substantially removing Means for receiving a digital data stream after transmission; Means (108) for extracting a copy of an echo cancellation reference signal from the received digital data stream; Means (106) for calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And Means (102, 104) for substantially canceling echo interference from the received digital data stream using the filter coefficients; The echo cancellation reference signal is non-cyclic, has a substantially flat frequency response within the bandwidth of the transmission path, has a large number of amplitude peaks in a time interval, and a shorter tail in proportion to the number of amplitude peaks. And substantially canceling echoes occurring during transmission of a digital data stream having a transmission frequency. The method of claim 4, wherein The echo cancellation reference signal is substantially a class of signal defined by the following equation, Wherein A, b and Ω are devices for substantially eliminating echoes occurring during transmission of a digital data stream which is a real number. The method of claim 5, A = 0.707 x 8.6 x 10 ^-6 volts; b = 0.9 x 10 ^-12 sec ² / radian; And an apparatus for substantially canceling echoes occurring during transmission of a digital data stream with Ω = 5.38 x 2πx 10 ⁶ radian / sec. The method of claim 4, wherein And said extracting means comprises means for extracting only one echo cancellation reference signal from between successive field syncs. The method of claim 4, wherein And said extracting means comprises means for extracting a plurality of echo cancellation reference signals from between successive field syncs. 10. An apparatus for modulating an ATSC VSB data stream prior to transmitting to the receiver to support an enhanced echo cancellation function of one receiver, wherein the ATSC VSB data stream comprises a plurality of field syncs (51, 54, 91, 94). And a plurality of symbol space segments (53, 56, 93, 96) between and subsequent field syncs, wherein the apparatus comprises means for inserting a plurality of echo cancellation reference signals between field syncs; The reference signal is substantially the class of the signal defined by the equation Wherein A, b and Ω are real numbers and modulate an ATSC VSB data stream before transmitting to the receiver to support the enhanced echo cancellation function of one receiver. The method of claim 9, Means for inserting blank symbol space for the echo cancellation reference signal further comprising: modulating an ATSC VSB data stream prior to transmitting to the receiver to support an enhanced echo cancellation function of one receiver. The method of claim 10, A = 0.707 x 8.6 x 10 ^-6 volts; b = 0.9 x 10 ^-12 sec ² / radian; And an apparatus for modulating an ATSC VSB data stream before transmitting to the receiver to support enhanced echo cancellation of one receiver with Ω = 5.38 x 2πx 10 ⁶ radian / sec. Multiple field syncs 51, 54, 91, 94 on the transmission path, multiple symbol space segments 53, 56, 93, 96 placed between successive field syncs, and multiple between the field syncs A method for substantially canceling echoes occurring during transmission of a digital data stream comprising echo cancellation reference signals 52, 55, 92, 95 of Receiving a digital data stream after transmission; Extracting a copy of the echo cancellation reference signal from the received digital data stream; Calculating filter coefficients from the extracted copy of the echo cancellation reference signal; And Substantially canceling echo interference from the received digital data stream using the coefficients; The echo cancellation reference signal is non-cyclic and has a substantially flat frequency response within the bandwidth of the transmission path, A method for substantially eliminating echoes occurring during transmission of a digital data stream having a large number of amplitude peaks in a time interval and having a shorter tail in proportion to the large number of amplitude peaks. The method of claim 12, The echo cancellation reference signal is substantially a class of signal defined by the following equation, Where A, b, and Ω are real methods for substantially eliminating echoes occurring during transmission of a digital data stream. The method of claim 13, A = 0.707 x 8.6 x 10 ^-6 volts; b = 0.9 x 10 ^-12 sec ² / radian; And a method for substantially eliminating echoes occurring during transmission of a digital data stream with Ω = 5.38 x 2π x 10 ⁶ radian / sec. The method of claim 12, And said extracting step comprises extracting only one echo cancellation reference signal from between successive field synchronizations. The method of claim 12, And said extracting step comprises extracting a plurality of echo cancellation reference signals from successive field synchronizations. 10. A method for modulating an ATSC VSB data stream prior to transmitting to the receiver to support enhanced echo cancellation in one receiver, wherein the ATSC VSB data stream comprises a plurality of field syncs (51, 54, 91, 94). And a plurality of symbol space segments (53, 56, 93, 96) between and subsequent field syncs, the method comprising a plurality of echo cancellation reference signals (52, 55, 92, 95) between field syncs. And inserting, wherein the echo cancellation reference signal is substantially a class of signal defined by Wherein A, b, and Ω are real numbers and modulate an ATSC VSB data stream before transmitting to the receiver to support enhanced echo cancellation in one receiver. The method of claim 17, The inserting step modulates an ATSC VSB data stream before transmitting to the receiver to support enhanced echo cancellation in one receiver, comprising inserting only one echo cancellation reference signal between successive field syncs. Way. The method of claim 17, The inserting step includes inserting a plurality of echo cancellation reference signals between successive field syncs to modulate an ATSC VSB data stream before transmitting to the receiver to support enhanced echo cancellation in one receiver. Way. The method of claim 17, Inserting only one echo cancellation reference signal between successive field syncs further comprising inserting a blank symbol space for the echo reference signal to support enhanced echo cancellation at one receiver. A method for modulating an ATSC VSB data stream prior to transmitting to the receiver.